[EH] Make rethrow's target a try label (#3568)

I was previously mistaken about `rethrow`'s argument rule and thought
it only counted `catch`'s depth. But it turns out it follows the same
rule `delegate`'s label: the immediate argument follows the same rule as
when computing branch labels, but it only can target `try` labels
(semantically it targets that `try`'s corresponding `catch`); otherwise
it will be a validation failure. Unlike `delegate`, `rethrow`'s label
denotes not where to rethrow, but which exception to rethrow. For
example,
```wasm
try $l0
catch ($l0)
  try $l1
  catch ($l1)
   rethrow $l0 ;; rethrow the exception caught by 'catch ($l0)'
  end
end
```

Refer to this comment for the more detailed informal semantics:
https://github.com/WebAssembly/exception-handling/issues/146#issuecomment-777714491

---

This also reverts some of `delegateTarget` -> `exceptionTarget` changes
done in #3562 in the validator. Label validation rules apply differently
for `delegate` and `rethrow` for try-catch. For example, this is valid:
```wasm
try $l0
  try
  delegate $l0
catch ($l0)
end
```
But this is NOT valid:
```wasm
try $l0
catch ($l0)
  try
  delegate $l0
end
```
So `try`'s label should be used within try-catch range (not catch-end
range) for `delegate`s.

But for the `rethrow` the rule is different. For example, this is valid:
```wasm
try $l0
catch ($l0)
  rethrow $l0
end
```
But this is NOT valid:
```wasm
try $l0
  rethrow $l0
catch ($l0)
end
```
So the `try`'s label should be used within catch-end range instead.

1 files changed, 106 insertions, 13 deletions

diff --git a/test/exception-handling.wast.fromBinary.noDebugInfo b/test/exception-handling.wast.fromBinary.noDebugInfo
index cf7372b6e..47577065c 100644
--- a/test/exception-handling.wast.fromBinary.noDebugInfo
+++ b/test/exception-handling.wast.fromBinary.noDebugInfo
@@ -194,19 +194,6 @@
     )
    )
   )
-  (try $label$37
-   (do
-    (throw $event$0
-     (i32.const 0)
-    )
-   )
-   (catch $event$0
-    (drop
-     (pop i32)
-    )
-    (rethrow 0)
-   )
-  )
  )
  (func $3
   (try $label$9
@@ -269,5 +256,111 @@
    (delegate 0)
   )
  )
+ (func $4
+  (try $label$3
+   (do
+    (call $0)
+   )
+   (catch $event$0
+    (drop
+     (pop i32)
+    )
+    (rethrow $label$3)
+   )
+   (catch_all
+    (rethrow $label$3)
+   )
+  )
+  (block $label$4
+   (try $label$7
+    (do
+     (call $0)
+    )
+    (catch $event$0
+     (drop
+      (pop i32)
+     )
+     (rethrow $label$7)
+    )
+    (catch_all
+     (br $label$4)
+    )
+   )
+  )
+  (try $label$13
+   (do
+    (call $0)
+   )
+   (catch_all
+    (try $label$12
+     (do
+      (call $0)
+     )
+     (catch $event$0
+      (drop
+       (pop i32)
+      )
+      (rethrow $label$13)
+     )
+     (catch_all
+      (rethrow $label$13)
+     )
+    )
+   )
+  )
+  (try $label$20
+   (do
+    (call $0)
+   )
+   (catch_all
+    (try $label$19
+     (do
+      (call $0)
+     )
+     (catch $event$0
+      (drop
+       (pop i32)
+      )
+      (block $label$18
+       (rethrow $label$20)
+      )
+     )
+     (catch_all
+      (rethrow $label$20)
+     )
+    )
+   )
+  )
+  (try $label$26
+   (do
+    (call $0)
+   )
+   (catch_all
+    (try $label$25
+     (do
+      (rethrow $label$26)
+     )
+     (catch_all
+      (nop)
+     )
+    )
+   )
+  )
+  (try $label$32
+   (do
+    (call $0)
+   )
+   (catch_all
+    (try $label$31
+     (do
+      (rethrow $label$32)
+     )
+     (catch_all
+      (nop)
+     )
+    )
+   )
+  )
+ )
 )